Monochrome light better for machine vision than white light

Black and white vs. color sensor? Monochrome or polychrome light frequencies? Visible or non-visible frequencies? Machine vision systems builders have a lot of choices – and options!

Let’s suppose you are working in the visible spectrum. You recall the rule of thumb to favor monochrome over color sensors when doing measurement applications – for same sized sensors.

So you’ve got a monochrome sensor that’s responsive in the range 380 – 700 nm. You put a suitable lens on your camera matched to the resolution requirements and figure “How easy, I can just use white light!”. You might have sufficient ambient light. Or you need supplemental LED lighting and choose white, since your target and sensor appear fine in white light – why overthink it? – you think.

Think again – monochrome may be better

Polychromatic (white) light is comprised of all the colors of the ROYGBIV visible spectrum – red, orange, yellow, green, blue, indigo, and violet – including all the hues within each of those segments of the visible spectrum. We humans perceive it as simple white light, but glass lenses and CMOS sensor pixels see things a bit differently.

Chromatic aberration is not your friend

Unless you are building prisms intended to separate white light into its constituent color groups, you’d prefer a lens that performs “perfectly” to focus light from the image onto the sensor, without introducing any loss or distortion.

Lens performance in all its aspects is a worthwhile topic in its own right, but for purposes of this short article, let’s discuss chromatic aberration. The key point is that when light passes through a lens, it refracts (bends) differently in correlation with the wavelength. For “coarse” applications it may not be noticeable; but trace amounts of arsenic in one’s coffee might go unnoticed too – inquiring minds want to understand when it starts to matter.

Take a look at the following two-part illustration and subsequent remarks.

Transverse and longitudinal chromatic aberration – Courtesy Edmund Optics

In the illustrations above:

  • C denotes red light at 656 nm
  • d denotes yellow light at 587 nm
  • F denotes blue light at 486 nm

Figure 1, showing transverse chromatic aberration, shows us that differing refraction patterns by wavelength shift the focal point(s). If a given point on your imaged object reflect or emits light in two more more of the wavelengths, the focal point of one might land in a different sensor pixel than the other, creating blur and confusion on how to resolve the point. One wants the optical system to honor the real world geometry as closely as possible – we don’t want a scatter plot generated if a single point could be attained.

Figure 2 shows longitudinal chromatic aberration, which is another way of telling the same story. The minimum blur spot is the span between whatever outermost rays correspond to wavelengths occurring in a given imaging instance.

We could go deeper, beyond single lenses to compound lenses; dig into advanced optics and how lens designers try to mitigate for chromatic aberration (since some users indeed want or need polychromatic light). But that’s for another day. The point here is that chromatic aberration exists, and it’s best avoided if one can.

So what’s the solution?

The good news is that a very easy way to completely overcome chromatic aberration is to use a single monochromatic wavelength! If your target object reflects or emits a given wavelength, to which your sensor is responsive, the lens will refract the light from a given point very precisely, with no wavelength-induced shifts.

Making it real

The illustration below shows that certain materials reflect certain wavelengths. Utilize such known properties to generate contrast essential for machine vision applications.

Red light reflects well from gold, copper, and silver – Courtesy CCS Inc.

In the illustration we see that blue light reflects well from silver (Ag) but not from copper (Cu) nor gold (Ag). Whereas red light reflects well from all three elements. The moral of the story is to use a wavelength that’s matched to what your application is looking for.

Takeaway – in a nutshell

Per the carpenter’s guidance to “measure twice – cut once”, approach each new application thoughtfully to optimize outcomes:

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Machine vision problems solved with SWIR lighting

Some problems best solved outside the visible spectrum

Most of us think about vision with a human bias, since most of us are normally sighted with color stereo vision. We perceive distance, hues, shading, and intensity, for materials that emit or reflect light in the wavelengths 380 – 750 nm. Many machine vision problems can also be solved using monochrome or color light and sensors in the visible spectrum.

Human visible light – marked VIS – is just a small portion of what sensors can detect – Courtesy Edmund Optics

Many applications are best solved or even only solved, in wavelengths that we cannot see with our own eyes. There are sensors that react to wavelengths in these other parts of the spectrum. Particularly interesting are short wave infrared (SWIR) and ultraviolet (UV). In this blog we focus on SWIR, with wavelengths in the range 0.9 – 1.7um.

Examples in SWIR space

The same apple with visible vs. SWIR lighting and sensors – Courtesy Effilux

Food processing and agricultural applications possible with SWIR. Consider the above images, where the visible image shows what appears to be a ripe apple in good condition. With SWIR imaging, a significant bruise is visible – as SWIR detects higher densities of water which render as black or dark grey. Supplier yields determine profits, losses, and reputations. Apple suppliers benefit by automated sorting of apples that will travel to grocery shelves vs. lightly bruised fruit that can be profitably juiced or sauced.

Even clear fluids in opaque bottles render dark in SWIR light –
Courtesy Effilux

Whether controlling the filling apparatus or quality controlling the nominally filled bottles, SWIR light and sensors can see through glass or opaque plastic bottles and render fluids dark while air renders white. The detection side of the application is solved!

Hyperspectral imaging

Yet another SWIR application is hyperspectral imaging. By identifying the spectral signature of every pixel in a scene, we can use light to discern the unique profile of substances. This in turn can identify the substance and permit object identification or process detection. Consider also multi-spectral imaging, an efficient sub-mode of hyperspectral imaging that only looks for certain bands sufficient to discern “all that’s needed”.

Multispectral and hyperspectral imaging – Courtesy Allied Vision Technologies

How to do SWIR imaging

The SWIR images shown above are pseudo-images, where pixel values in the SWIR spectrum have been re-mapped into the visible spectrum along grey levels. But that’s just to help our understanding, as an automated machine vision application doesn’t need to show an image to a human operator.

In machine vision, an algorithm on the host PC interprets the pixel values to identify features and make actionable determinations. Such as “move apple to juicer” or “continue filling bottle”.

Components for SWIR imaging

SWIR sensors and cameras; SWIR lighting, and SWIR lenses. For cameras and sensors, consider Allied Vision’s Goldeye series:

Goldeye SWIR cameras – Courtesy Allied Vision

Goldeye SWIR cameras are available in compact, rugged, industrial models, or as advanced scientific versions. The former has optional thermal electric cooling (TEC), while the latter is only available in cooled versions.

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For SWIR lighting, consider Effilux bar and ring lights. Effilux lights come in various wavelengths for both the visible and SWIR applications. Contact us to discuss SWIR lighting options.

EFFI-FLEX bar light and EFFI-RING ring light – Courtesy Effilux

By emitting light in the SWIR range, directed to reflect off targets known to reveal features in the SWIR spectrum, one builds the components necessary for a successful application.

Hyperspectral bar lights – Courtesy Effilux

And don’t forget the lens. One may also need a SWIR-specific lens, or a hybrid machine vision lens that passes both visible and SWIR wavelengths. Consider Computar VISWIR Lite Series Lenses or their VISWIR Hyper-APO Series Lenses. It’s beyond the scope of this short blog to go into SWIR lensing. Read our recent blog on Wide Band SWIR Lensing and Applications or speak with your lensing professional to be sure you get the right lens.


Whether SWIR or UV (more on that another time), the key point is that some machine vision problems are best solved outside the human visible portions of the spectrum. While there are innovative users and manufacturers continuing to push the boundaries – these areas are sufficiently mature that solutions are predictably creatable. Think beyond the visible constraints!

Call us at 978-474-0044. Or follow the contact us link below to provide your information, and we’ll call you.

1st Vision’s sales engineers have over 100 years of combined experience to assist in your camera and components selection.  With a large portfolio of lensescablesNIC cards and industrial computers, we can provide a full vision solution!

Optimize wavelength to maximize contrast

Seasoned machine vision practitioners know that while the sensor and the optics are important, so too is lighting design. Unless an application is so “easy” that ambient light is enough, many or most value-added applications require supplemental light. “White” light is what comes to mind first, since it’s what we humans experience most. But narrow-band light – whether colored light within the visible spectrum, or non-visible light with a sensor attuned to those frequencies – is sometimes the key to maximizing contrast.

Gold contrasts better with red light than either white or blue – Image courtesy of CCS America

In the illustrations above, suppose we have an application to do feature identification for gold contacts. The ideal contrast to create is where gold features “pop” and everything that’s not gold fails to appear at all, or at most very faintly. If the targets that will come into the field of view have known properties, one can often do lighting design to achieve precisely such optimal outcomes

In this example, consider the white light image in the top left, and then the over-under images created with red and blue light respectively. The white light image shows “everything” but doesn’t really isolate the gold components. The red light does a great job showing just the gold (Au). The blue light emphasizes silver (Ag). The graph to the right shows four common metals relative to how they respond under which (visible) wavelengths. Good to know!

For an illustrated nine-page treatment of how various wavelengths improve contrast for specific materials or applications, download this Wavelength Guide from our Knowledge Base. You may be able to self-diagnose the wavelength ideal for your application. Or you may prefer to just call us at 978-474-0044, and we can guide you to a solution.

To the left we see 5 plastic contact lens packages, in white light. Presence/absence detection is inconclusive. Image courtesy of CCS America.

With UV light, a presence/absence quality control check can be programmed based on a rule that presence = 30% or more of the area in each round renders as black. Image courtesy of CCS America.

It all comes down to the reflection or absorption characteristics of specific properties with respect to certain wavelengths. Below we see a chart showing the peaks of some of the more commonly used wavelengths in machine vision.

Commonly used wavelengths – Image courtesy CCS America

For more details on enhancing contrast via lighting at specific wavelengths, download this Wavelength Guide from our Knowledge Base. Or click on Contact Us so we can discuss your application and guide you. 1stVision has several partners with different lighting geometries and wavelengths to create contrast. All three partners are in the same business group. CCS America and Effilux offer a variety of wavelengths (UV through NIR) and light formats (ring light, back light, bar light, dome). Gardasoft has full offerings for lighting controls. Tell us about your application and we’ll help you design an optimal solution.

1st Vision’s sales engineers have over 100 years of combined experience to assist in your camera and components selection.  With a large portfolio of lensescablesNIC card and industrial computers, we can provide a full vision solution!

EFFI Flex2 Lights – What’s New? 

EFFI-Flex2 LED bar lights

EFFI Flex2 LED bar lights are the next generation modular lights with even more flexibility. 

Series 1 vs. EFFI-Flex2 NextGen Series – courtesy of Effilux

Getting the lighting right is as important as choosing the right sensor, camera, and interface, in terms of machine vision success. Most all lights are “pre-configured” to a specific lighting geometry as well as wavelengths.  Lighting is still tricky and in most cases, testing is needed with various configurations. If your samples are small in size and don’t require special handling or equipment, your imaging partner may offer a lighting lab in which they can test your samples to optimize lighting configuration.

But shipping samples and re-creating your conditions in a partner’s lab isn’t always practical, and you may want to do your own testing and configuration, taking advantage of your domain expertise and home-field advantage. With EFFI Flex2 lights, the standard components and range of settings yield 36 configurations in 1 light! How so? Each unit comes with 3 diffuser windows x 4 lens positions x 3 electronic modes = 36 configurations. Optional additional accessories, like polarizers, take the calculation to more than 100 configurations!

EFFI Flex2 LED modular bar lights are configurable to a wide range of applications.  The user can adapt the optics, the electronics, and the mechanics, thanks to the engineering design.  Perhaps you’ll embed the unit in a “forever” deployment mode, never again re-configuring a specific unit once you lock down the optimal settings.

Or maybe you’ll adapt your light again to repurpose it in another application. With the long service life of LED lights, the light may well outlive the application. Or maybe you do multiple applications, and want a light that’s so versatile you can do all your testing in house, letting the lighting drive the final choice of sensor and camera model.

As with the first generation EFFI-Flex product, EFFI-Flex2 series is designed to provide easy adaptation of:

  • Optics – adjust the lens position and the diffusers
  • Electronics – built-in multimode driver offers 3 operating modes in one light
  • Mechanics – optical lengths from 60mm – 2900mm (factory configured)
One-minute video shows key optical and electronic configuration options

Optically,  lens positions may be user-adjusted for emission angles ranging from from 90 to 10 degrees. Each unit ships with swappable diffusers for strobed light, diffused, semi-diffused, or opaline light. Further, if necessary for your particular application, optional optical accessories are available: polarizer, linescan film, and cylindrical lens).

Electronically, the built-in multimode driver features 3 electronic modes: AutoStrobe with 450% Overdrive, Adjustable Strobe with controlled intensity from 10% to 100%, and Dimmable Continuous with controlled intensity from 10% to 100%. The 450% Overdrive mode is 1.5 times more powerful than the original EFFI-Flex LED bar light in overdrive.

The driver software makes it easy to select among the 3 modes, and the parameters within each mode.

Mechanics: While the length of a given unit cannot be adapted once delivered, one may order in lengths from as short at 60mm to as long as 2900mm. If your default units are English rather than metric, that’s from less than 3 inches to as much as 9.5 feet!

EFFI-Flex original series remains in production. If you don’t require the flexibility of EFFI-Flex2, with up to 36 configurations per unit shipped, the original series offers great value in its own right. Call us at 978-474-0044 to speak with one of our sales engineers, for guidance, or take out your own appendix with this side-by-side comparison diagram:

Series 1 vs. NextGen EFFI-Flex2 – courtesy of Effilux

1st Vision’s sales engineers have over 100 years of combined experience to assist in your camera and components selection.  With a large portfolio of lensescablesNIC card and industrial computers, we can provide a full vision solution!